The knob was a `MakeBlur` image filter on the draw of the graded photo — one
sigma over all three channels — so at NOISE REDUCTION 100 a 1024-pixel preview
lost every edge finer than 0.6 of a pixel of its own and nothing brought the
luminance detail back. That is thay_doi_thong_so_giong_lightroom.md §4's own
warning ("Noise Reduction sẽ làm nhòe toàn bộ chi tiết sợi tóc và vân da") written
into the engine, and the filter had a second cost: a paint filter is handed the
shader's INPUT, so the pass could never read the graded pixels it was supposed to
correct.
It is a two-child pass now, NR_SKSL, run after the draw: the frame, and a blurred
copy of it (blurredFrame — the snapshot read back through the same MakeBlur the
ramp's base and the negative sharpening use). The output takes its CHROMA from
the blurred child and its LUMA from the frame, the split the tone ramp's header
already describes (`rgb - luma`), so the output's brightness is the input's at
every amount by construction — the eye is nearly blind to a hue change at that
scale, which is the whole reason the colour half is free. The reference reaches
NR_CHROMA_SPAN = 0.4% of the frame's width, the doc's own 3..5 pixels of a
full-resolution frame, where the knob's blur was 0.6 of a pixel.
The luminance half of §4.1 (its bilateral filter) is deliberately not here: it is
the half that costs detail and no frame has shown grain the colour half left
behind. ponytail: add it as a second child of this same pass when one does.
Checked: `tsc --noEmit` clean; `denoise-check.mjs` (new) compiles NR_SKSL on
CanvasKit, asserts the engine still wires both children and no longer blurs the
draw, and renders the pass at five amounts against a flat pair — amount 0 is the
pixel exactly, amount 1 carries the neighbourhood's colour difference, and the
luma never moves at any of them; `highlight-knee-check.mjs`, `tone-base-check.mjs`
and `mask-wb-check.mjs` still green.
